The present invention relates to an operational amplifier circuit, and more particularly to an improved front end stage of an operational amplifier having increased transconductance gain and improved slew rate characteristics.
Generally, the output current of the front end stage corresponds to a hyperbolic tangent with the input differential voltage. However, for input voltages greater than a certain magnitude, slew rate limiting occurs with the output current being equal in magnitude to that of the current sources for the front end stage, thereby resulting in no further increase in output current with increases in input differential voltages. Although various techniques have been developed to improve the slew rate and output capabilities, these techniques adversely effect other related characteristics of the amplifier such as transconductance gain, offset voltages, and temperature coefficients.
Accordingly, it is an object of this invention to provide an improved amplifier with a improved slew rate without adversely effecting the other characteristics of the front end stage of the amplifier.
It is further an object of this invention to provide an improved amplifier with high gain capabilities.
It is another object to provide a new and useful technique for increasing both the slew rate and current output capabilities of the front end stage without adversely effecting other related characteristics of the amplifier.
These and other objects are attained by providing a large signal or slew enhancement stage connected in parallel to a small signal or front end stage of an operational amplifier, thereby expanding both the slew rate and transconductance gain of the amplifier.
The small signal stage includes at least one differential transistor pair, with each pair having a differential input and a single output. The large signal stage includes a complementary set of differential transistor pairs, in addition to a pair of active loads and output means, the output means being switched on and off by the active loads.
Depending upon the polarity of the input differential voltage, the small signal stage functions as either a high gain current source or current sink for a subsequent stage of the amplifier. Until the small signal stage reaches its slew rate limit; the active loads of the large signal stage will maintain the respective output means in a nonconductive state, such that no additional current is provided to the front end stage by the large signal stage. Upon the small signal stage reaching its slew rate limit, however, one of the active loads of the large signal stage will bias its respective output means to conduct, thereby allowing this second stage to either serve as as additional current source or current sink to the output of the front end stage. Which active load and output means will conduct depends upon the polarity of the differential voltage applied to the parallel inputs of the differential transistor pairs.
These and other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.